Automatic bias control



Jam. 10, 1939. v v R. B. DOME I 2,143,589

AUTOMAT IC B IAS CONTROL Filed May 9, 1936 Inventor: RobeTtB. Dome,

His Attorney.

Patented Jan. 10, 1939 UNITED STATES PATENT OFFICE AUTOMATIC BIAS CONTROL New York Application May 9, 1936, Serial No. 78,949

7 Claims.

My invention relates to energizing circuits for electron discharge devices and more particularly to such circuits employed in electron discharge amplifiers.

It is well known that electron discharge devices having a fixed grid bias cannot operate at optimum efficiency, that is, with minimum anode power consumption, over widely varying signal intensity levels. While this lack of automatic control of the operating point of an electron discharge device is not a serious one in ordinary broadcast receivers designed to operate on available public service alternating current supply, it becomes an important consideration when a receiver is designed for battery operation. It is an object of my invention to provide a novel means'for automatically varying the grid biasing potential in response to variations in the signal intensity level.

It is a further object of my invention to provide novel means for causing an electron discharge device to operate at substantially its point of maximum efficiency irrespective of variations in signal intensity.

The objects of my invention are attained through the utilization of one or more resistors having a non-linear resistance-ampere characteristic which .are interposed in the energizing circuit of an electron discharge device. A material which satisfactorily fulfills this requirement is commercially known as Thyrite, and is described in a United States patent to McEachron No. 1,882,742. This material has a substantially hyperbolic resistance-ampere characteristic.

Thyrite is a material somewhat similar to dry process porcelain throughout the mass of which appear minute particles of conducting material. The substance is substantially an insulator at low potentials and becomes an increasingly better conductor as the current therethrough is increased. For a better understanding of the material itself and its characteristics reference may be made to the above identified United States patent and to an article entitled Thyrite; a new material for lightning arresters by K. B. McEachron, appearing on page 92 of the General Electric Review for February, 1930.

It should be noted that the material Thyrite is usually utilized in the form of disks each face of which is provided with a sprayed metallic coating for contact purposes. The shape, however, of the material is not at all controlling and I in fact it may be utilized in any mouldable shape.

The resistance of Thyrite varies directly with its thickness but not inversely with its area as does the resistance of materials such as carbon or other metals which have heretofore commonly been utilized.

The novel features which I believe tobe characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which the single figure thereof represents one embodiment of my invention.

Referring now to the drawing, I have shown therein a single stage amplifier including an electron discharge device I, having an anode 2, a cathode 3, and a grid electrode 4. A suitable source of unidirectional current such as a battery 5 is connected in series with primary winding 6 of load transformer I between anode 2 and cathode 3, the positive side of battery 5 being of course on the anode side. A source of signal energy 8,

which may be one of audio frequency, is coupled to grid electrode 4 by means of a coupling condenser 9. The positive side of a second source of unidirectional current such as battery I!) is connected to cathode 3. A resistor II having a nonlinear resistance-ampere characteristic such, for;

example, as Thyrite and a resistor I2 of the usual type are connected in series across battery ID. Grid electrode 4 of discharge device I is connected to apoint between resistor I I and resistor I2 through a grid resistor I3 and a filter resistor I4. The usual filter capacitor I5 is connected between cathode 3 and the grid resistor side of filter resistor I4. A control coupling capacitor I B and a control voltage regulating resistor I! are connected between anode 2 and the lower side of resistor II as shown.

It will thus be seen that a path is provided for alternating currents flowing through discharge device I which in addition to device I includes resistor II, resistor I1 and capacitor I6. It will be observed that resistor II is also included in a path for unidirectional currents flowing from battery II], which path includes resistor II and resistor I2.

While circuit constants may be chosen within wide limits, I have found, for example, that very satisfactory results may be obtained by adjusting the circuit with nosignal by varying the value of resistor I2 until the anode current has a value of approximately one-fourth of the value regmaese quired for full output and then with full signal impressed on grid electrode 4 the value of resistor I1 is adjusted until electron discharge device l is operating with normal anode current;

The variation of resistance l1, would, of course, have no effect upon the voltage across resistance H were it not for the high value of resistance [2. That is, were resistance l2 not high the potential of source l would be eifective across resistance H and would tend to establish a fixed potential across resistance Ii independent of alternating current flowing through condenser [5. Since this resistance I2 is high, the unidirectional potential is smaller than the alternating potential impressed thereon and the potential on re-' sistance ll varies downwardly in value substantially during each half-wave of alternating current. This does not materially effect the direct prising elements It, i! and H the resistance of element H drops. This decrease in resistance lowers that portion of the bias potential from source It! which appears across resistor II. Inasmuch as the potential drop across resistor ll L is the biasing potential of discharge device I,

the bias is lowered with a. resulting increase of anode current. This change in bias may, by proper choice of values for the different circuit constants be of such magnitude as to causethe discharge device to operate continuously on the portion of its grid bias anode current. characteristic where maximum efficiency is obtained.

Filter resistor M and filter capacitor [5 permit the current in resistance l to build up fast enough to avoid chopping oif the front portion of words or sounds and yet slow enough so that the alternating current electromotive forces appearing on resistance 5 l are prevented from being impressed on grid electrode 4 where their presence would cause a certain amount of degeneration.

, When source 8 has relatively low internal resistance such as would be the case for example if source 8 were a diode or a lowresistance amplifier tube, resistor l4, and condenser it: may be omitted. This is by reason of the fact that degeneration in such cases is negligible (the resistance of resistor l3 being many times that of source 8) because the portion of the alternating current disturbance across resistor H which gets to the grid is only part of the whole.

While I have shown a particular embodiment ofmy invention, it will of course be understood that I do not wish to be limited thereto since many modifications may be made both in the circuit arrangement and inthe instrumentalities employed, andI therefore contemplate byv the appended claims to cover all such modifications as'fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The combination comprising an electron dis charge device, an input circuit therefor including a source of signal energy, an output circuit, means for continuously operating said discharge device on the portion of its grid bias anode current characteristic where maximum efliciency is obtained, said means including a path for unidirectional current between the grid and cathode of said discharge device, said path including a grid bias resistance, and means for automatically varying the average value of said grid bias resistance in response to variations in average intensity of said signal energy.

2. In combination, an electron discharge device having an anode, a cathode, and a grid electrode, an input circuit including a source of signal energy connected between said grid electrode and said cathode, an output circuit including a source of unidirectional current connected between said anode and said cathode, means for impressing a biasing potential on said grid electrode, said means comprising a bias resistance connected between said grid electrode and cathode, and means for automatically decreasing the average value of said bias resistance in response to an increase in the average intensity of said signal energy by an amount sufiicient to vary the point on the grid bias anode current characteristic of said device where operation occurs sufficiently to maintain maximum efiiciency of operation of said discharge device.

3. In combination, an electron discharge device having an anode, a cathode, and a grid electrode,

a source of signal energy connected to said grid electrode and to said cathode, biasing means including a source of unidirectional current for impressing a biasing potential on said grid electrode, an output circuit including a source of unidirectional current connected between said anode and said cathode, an alternating current circuit connected in parallel with said output circuit, and a resistor having a non-linear resistanceampere characteristic, said resistor being common to both the circuit of said biasing means and to said alternating current circuit.

4. In combination, an electron discharge device having an anode, a cathode and a grid electrode, a source of signal energy connected to said grid electrode and to said cathode, biasing means including a source of unidirectional current and a resistor having a non-linear resistance-ampere characteristic for impressing a biasing potential on said grid electrode, an output circuit including a source of unidirectional current connected between said anode and said cathode, means adapted to pass the alternating anode-cathode current of said discharge device through said resistor, thereby automatically to vary the biasing potential on said grid electrode in response to variations in intensity of said signal energy, and means to prevent the alternating component of electromotive force on said non-linear resistor from being impressed on said grid.

5. In a voltage divider circuit, the combination of a source of unidirectional current, a resistor having a substantially linear resistance-ampere characteristic and a second resistor having a non-linear resistance-ampere characteristic connected across said source of unidirectional current, an output circuit adapted to have impressed thereon the steady unidirectional component of voltage across said second resistor caused by the flow of unidirectional current therethrough, am

source of alternating current energy of varying intensity, means for causing alternating current from said alternating current source to flow through said second resistor with an amplitude greater than the value of unidirectional current in said second resistance, thereby to decrease said steady unidirectional component of voltage across said second resistance in accordance with the average intensity of said alternating electromotive force, and means to prevent the alternating component of electromotive force on said second resistance from being supplied to said output cir cuit.

6. In combination, an electron discharge device having an anode, a cathode and a grid, a nonlinear resistor, means to' transmit a unidirectional current through said non-linear resistor, said non-linear resistor being connected in a direct current path between said grid and cathode, means to pass an alternating current through said non-linear resistor having an intensity dependent upon the alternating electromotive force on said anode and greater than the intensity of direct current therethrough thereby to vary the unidirectional potential on said resistance, and means to prevent the alternating component of electromotive force developed on said resistor by said alternating current from being effective between said grid and cathode.

'7. In combination, an electron discharge device having an anode, a cathode, and a grid, a circuit between said anode and cathode including a source of anode operating potential, a grid bias circuit between said grid and cathode including a source of grid bias potential and a high resistance, a non-linear resistance connected between said cathode and the grid terminal of said high resistance, and means to pass alternating current from said anode-cathode circuit through said non-linear resistance.

ROBERT E. DOME. 

